[test] Add new CLI testing platform

Adds the new CLI testing platform that I'm proposing.
See the added `README.md` for details.

tests/cli-tests/compression/multi-threaded.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -e
+
+# Test multi-threaded flags
+zstd --single-thread file -f            ; zstd -t file.zst
+zstd -T2 -f file                        ; zstd -t file.zst
+zstd --rsyncable -f file                ; zstd -t file.zst
+zstd -T0 -f file                        ; zstd -t file.zst
+zstd -T0 --auto-threads=logical -f file ; zstd -t file.zst
+zstd -T0 --auto-threads=physical -f file; zstd -t file.zst